TIM: a model protein for the study of crystal growth in space and gels

The understanding of the basic processes underlying the crystallisation of proteins requires the use of very well-characterised model systems, for which theory and models of crystal growth can be verified experimentally. Triose phosphate isomerase (Tm TIM) [1] can be produced in large amounts, and crystallises reproducibly. It has been used for experiments in microgravity and in gels, and it has been shown to produce crystals of marked improvement in these non-convective environments. It is thus an interesting model protein for studying the effect of convective mixing on protein crystallisation. We present here the phase diagrams for Tm TIM in function of temperature, protein, and precipitant concentration, and relate the results with the observed crystallisation behaviour. Solubility curves and the binodal curve in function of these tree parameters show that Tm TIM is a case of ‘retrograde solubility’. This phenomenon has been observed in many cases but only been described in detail in the case of human hemoblobin [2]. The phase diagrams form the basis for the understanding of the results from counterdiffusion experiments with Tm TIM, including improved crystal quality, and polymorfic variation in function of growth conditions.